While I will have to defer to you that it is mechanics, it still seems hit and miss. I did get in to the room today after leveling up party to level 10 and upgrading to six mechanics for my Ranger, I would have sworn that I had tried it several times before using rest bonus to get to 6. Adding to the confusion for the first time I tried wearing the cloak of Lilith -- which of course adds +3 to perception -- to further befuddle me.
Just thought there was something else I was missing. Quite happy just muddling through most everything, but finding these sword pieces is dragging me down. Now I can't find the switch on 9th level for the hidden room. Resting gets me to Mech 7. I'm off to try again. If not I'll go bounty hunting.